I know topic has been covered a lot on this forum, but I have read through many of the previous posts and have not been able to solve my problem. I am using a 12V, 4 pin (+12, ground, sense, control) Arctic F9 PWM fan (specs: http://formfactors.org/developer%5Cspecs%5CREV1_2_Public.pdf) with my Arduino Uno. So far, I have been able to somewhat control the fan speed by varying the duty cycle on the PWM pin, but I would also like to be able to measure and output the fan speed in RPMs. I have attached a jpg of the the setup I've been using. I also verified that the sense pin is outputting 5V in this setup using a volt meter. I have tried several codes from previous posts (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1152891511/0), but the only thing I am able to get out of it is 0 rpm, no matter what the fan speed actually is. I need to be able to accurately read the value across the entire fan speed range of 600 to 1800 RPM. I've only been using Arduino for a few weeks, so I'll admit that I'm pretty ignorant about this stuff but any help, hardware or software related, would be much appreciated.